I asked that very question on Space.SE: https://space.stackexchange.com/questions/6256/why-were-jet-engines-not-used-in-place-of-the-solid-fuel-in-the-sts-srbs

I asked that very question on Space.SE: https://space.stackexchange.com/questions/6256/why-were-jet-engines-not-used-in-place-of-the-solid-fuel-in-the-sts-srbs https://space.stackexchange.com/questions/6256/why-were-jet-... The summary is that there isn't enough oxygen at the range of altitudes needed.